Experimental and clinical observations have supported the concept that neuropeptides play central roles in neurotransmission as well as the regulation of secretory functions of adenohypophysial, pancreatic, adrenalcortical and gut cells. Among the thirty or so neuropeptides that have been implicated in neuronal function in the mammalian central nervous system, several have also been suggested to function as neurotransmitters or neuromodulators primarily in afferant neurons. It has been suggested that the application of recombinant DNA technology to the analysis of mRNAs of the brain or other neural tissues may provide a means for identifying new transmitter substances and their related proteins. Thus, following the inferential discovery of novel brain and other neuropeptide transmitters via DNA sequencing technologies, it is possible to synthesize the predicted structure of the new peptide, generate antibody to this peptide and establish its existence, in various neural tissues by classical immunohistochemical procedures. Furthermore, the synthetic peptide itself can be used as a source for identifying and investigating its physiological actions.
Calcitonin is a 32-residue, amidated peptide hormone which was earlier isolated and characterized. Human calcitonin is found in the thyroid and possibly in the pituitary and has the formula: ##STR1## Calcitonin has biological activity in lowering body calcium levels and in promoting absorption of calcium into bone.
The diagnosis of human medullary thyroid carcinoma is primarily based on the presence of high circulating levels of serum calcitonin detected by radio-immunoassay (RIA) or other immuno-diagnostic procedures utilizing anti-calcitonin antibody. Unfortunately, a high percentage of individuals with medullary thyroid carcinoma do not present high calcitonin serum levels. An alternative product expressed by the calcitonin gene, termed CGRP, is a peptide that can be synthesized and secreted by medullary thyroid carcinoma cells. Thus, the production of CGRP and its identification by immuno-diagnostic procedures utilizing anti-CGRP antibody should be valuable in the diagnosis of human medullary thyroid disease.